The electrical monitoring system described herein relates to monitoring and/or control of changes within an electrical power grid. In particular, the electrical monitoring system concerns, at least in part, the detection and communication of data descriptive of such disturbances.
In the United States and most other countries, generating systems producing electrical power are typically interconnected in a power grid by high voltage alternating current (AC) three-phase electric power transmission lines. One reason power generating systems and their loads are interconnected with other power generating systems and their loads is to provide greater reliability in relation to cost. Interconnecting neighboring utilities through transmission systems allows effective sharing of reserves. As a result, each utility requires a smaller percentage reserve margin at a given level of reliability. Moreover, extensive interconnection of power systems may allow for greater use of generators that have lower operating costs.
However, there are also liabilities associated with larger size and interconnection of power systems. Long transmission lines introduce the problem of stability in addition to the problem of transmission power loss and expense of land acquisition, construction and maintenance. More interdependence among areas also means greater vulnerability to events far away, such as high loads, or generator outages, or transmission line faults, that can cause voltage and frequency fluctuations.
Optimization of the distribution of power throughout a power grid presents a non-trivial calculation. Generally data from various locations on the power grid are collected and analyzed with sophisticated computing systems, with the results being used to assess the performance of the grid. Depending upon the assessment, responses can be made, for examples, to add certain generating capacity, to remove other generating capacity, to disconnect various transmission lines, or to take such other action as may be appropriate. Typically, the computations are based in part on the monitored quantities of real and reactive power injected or consumed at various locations.
The modeling, monitoring and controlling process is made more difficult by the changing economic parameters governing electrical power generation, transmission, and distribution. One approach to address this issue of streamlining or automating the operation of transmission and distribution systems involves the obtaining of ever increasing amounts of data by remote sensing. However, there are often limits of reliable bandwidth capacity to and from the remote locations that make such an approach expensive or prohibitive.
During small time intervals, it is possible to store high resolution digitized waveforms and related information about transmission line voltages and currents, and to then retain that stored data long term when an anomalous condition arises. Unfortunately, data storage resources limit the amount of data that may be collected in this manner, and bandwidth cost and limitations inhibit remote access to this information in real time. The desire to capture data for a rapid succession of anomalous conditions also limits the amount of storage that may be allocated to a given data collection event. Moreover, real time transmission of data to centralized control systems requires a reliable substantial network and centralized processing capacity. Sometimes broadband connections to remote locations have reliability problems that correlate to the same causes of interruptions of electrical power in the grid, such as hurricanes, for example. There are typically more options for narrowband connections, and they may be more reliable and lower cost, yet they can not provide to remote locations the full amount of data that can be collected locally. Early detection and characterization of changes in an electrical power grid are desirable.
The line disturbance monitor shown in the patent to Bagnall et al., U.S. Pat. No. 4,484,290 receives analog signals representing voltage and current of transmission power lines and converts them to a digital representation. The monitor includes storage means for sequentially generated digital data representative of the sampled AC voltages and currents.
Phadke et al. have described a method of obtaining voltage phasors for use in detecting line disturbances which involves a recursive computation for the real and imaginary phasor components. This technique is discussed in “A New Measurement Technique for Tracking Voltage Phasors, Local System Frequency, and Rate of Change Frequency,” IEEE Paper No. 82, SM 444-8, A. G. Phadke, J. S. Thorpe, and M. G. Adamiak (1982). In the Phadke et al. approach, a recursive equation is used to determine the phasor representation of the power line voltage based on digitized signal data. This phasor is subsequently used to calculate AC operating parameters such as phase angle, positive sequence voltage, and line synchronization parameters using a microprocessor-based routine.
U.S. Pat. No. 5,216,621 to Dickens is a source of additional information concerning one waveform storage approach and is hereby incorporated by reference. The '621 Dickens patent discloses a fault detection system for monitoring at least one operating parameter of an AC power transmission line including means connectable to the transmission line for providing an analog signal representative of the time varying value of the operating parameter. An analog-to-digital (A/D) converter coupled to an input means samples the analog signal and produces digital sample data representing the signal.
U.S. Pat. No. 6,415,244 to Dickens et al. is also incorporated by reference herein. The '244 Dickens et al. patent discloses other techniques to monitor electrical power by generating multiple data streams representative of the electrical power. The data streams are further processed to selectively record data associated with power anomalies, disturbances, or other events of interest. The record may include at least a pre-fault portion and a post-fault portion of each of the first or second data streams.
There remains a need for an improved electrical monitoring and control system for an electrical power grid that collects data from several remote locations. Moreover, there is a need for new power grid management techniques that provide remote digital disturbance monitoring for AC transmission lines to create situational awareness of power grid stability without overwhelming the data sampling and processing capabilities of electrical power generation and distribution provider. This includes the need to develop a monitoring system and techniques that can be used without precise synchronization of local time bases between remote locations, allowing for only general references to the line frequency itself, or other imprecise methods of synchronization, and avoiding cost and/or reliability issues that can be associated with some precise synchronization approaches.
Another pressing need is to provide techniques to create a robust, widely distributed and secure data measurement infrastructure over an electrical power grid with associated analysis and monitoring tools for better planning, operation, and improved reliability. Other benefits, improvements and solutions offered by the electrical power sensing and control system will become apparent from the following written description and accompanying drawings.